Traffic signals have long been used to regulate the flow of traffic at intersections. Generally, traffic signals have relied on timers or vehicle sensors to determine when to change the phase of traffic signal lights, thereby signaling alternating directions of traffic to stop, and others to proceed. This situation is commonly exemplified in an emergency-vehicle application.
Emergency vehicles, such as police cars, fire trucks and ambulances, are generally permitted to cross an intersection against a traffic signal. Emergency vehicles have typically depended on horns, sirens and flashing lights to alert other drivers approaching the intersection that an emergency vehicle intends to cross the intersection. However, due to hearing impairment, air conditioning, audio systems and other distractions, often the driver of a vehicle approaching an intersection will not be aware of a warning being emitted by an approaching emergency vehicle.
There are presently a number of known optical traffic priority systems that permit for a fixed code to be embedded into the data stream to identify each vehicle and provide security. Such a code can be compared to a list of authorized codes at the intersection to restrict access by unauthorized users. This approach can be disadvantageous for certain applications or environments. For example, one problem with this approach arises when the transmitted data protocol is generally known or can easily be intercepted and re-created by unauthorized users. Once the transmitted data has been decoded or the transmitted data has been recorded for future playback, an unauthorized device can be used to activate the system. In addition, an unauthorized device can be used to activate the system without intercepting any transmitted data by attempting to activate the system using various codes until a code is discovered that successfully activates the system.
There are some straight-forward approaches for preventing such unauthorized access to the traffic light control systems. One approach is to remove any such intercepted or discovered code from the system database altogether. Coordination of such removal, however, can be burdensome and expensive since the vehicle code and the authorized code list at each intersection would need to be changed. Another approach is to prevent the unauthorized use by equipping all authorized vehicles, as well as the intersection (traffic light control) systems, with special communication transceivers that interact to provide another layer of security before providing access to the traffic light control systems. This approach can also be burdensome and expensive since each of the vehicles, as well as the systems at each intersection, would need additional equipment.